This invention relates to so-called "carbonless copying record" systems, in particular to those systems which employ an electron donor-acceptor reaction between particles of a solid electron acceptor distributed on image-receiving substrates and an organic solution of a colorless or substantially colorless basic dye precursor on transfer substrates. The invention is directed to improvements in image-receiving substrates of the type in which reactive inorganic pigment particles coated on the surface develop solutions of basic dye precursors such as crystal violet lactone which normally form fugitive images when developed on substrates sensitized with prior art acidic inorganic pigments.
Several varieties of commercial carbonless copying paper utilize the chemical reaction between particles of a solid acid and a colorless or substantially colorless basic chromogenic compound to form a printed image. Usually the dye precursor is encapsulated in small pressure-rupturable capsules (microcapsules) which are coated on transfer surfaces of paper (CB) and the particles of acid solid are coated on a recording area of the reverse (upper) side (CF) of the sheet. In a simple business form where the object is to obtain a single copy, an upper sheet bearing a microcapsule coating (CB) on its lower side is stacked above a lower sheet bearing the acidic solid (SF) on its upper side. Upon application of printing pressures to the top of the upper (CB) sheet of the assembly, the capsules are ruptured and the dye solution is transferred from the CB to the CF sheet, thereby forming colored printed marks on the CF sheet.
In a so-called multiple business form, where the object is to obtain several copies, a multiplicity of sheets bearing an acidic solid on the upper side and a microcapsule coating on the lower side (CFB) are stacked between the uppermost CB sheet and the CF sheet which is placed at the bottom of the stack. In this case the application of pressure ruptures microcapsules in the CB coatings of the uppermost sheet and also in those of the inner CFB sheets. Color precursor is transferred from the CB coatings to the CF coating of the next sheet below, whereon a colored mark appears by the process described above. It should be noted that in commercial practice, frequently the uppermost sheet of a single or multiple form will not be a CB sheet as described, but a CFB. The reason for using a CFB where only a CB is required lies in the economics of the paper coating process and derives from the relatively low cost of CF material as opposed to the relatively expensive CB materials. However, in order for the system to function adequately in the sense of providing the copies desired, only a CB coating is required on the uppermost sheet of the form. Similarly it would be permissible though not necessary to use a CFB as the bottom sheet in the form. However, this occurs less frequently in practice due to the aforementioned high cost of the CB coating materials, since only a CF coating is required on the bottom sheet in order that the system should function.
It has also been proposed to provide self-contained record material in which the solution of dye precursor and acidic particles are on the same side of a member but normally separated by a barrier such as encapsulating material.
In the early stages of the development of carbonless copying paper systems, the image-developing receiving members were provided by applying an aqueous coating composition containing a paper coating adhesive and particles of attapulgite clay, a unique clay material. These CF members were assembled with transfer members coated with an encapsulated oily solution containing both a primary dye precursor, crystal violet lactone (CVL), and a secondary dye pecursor, benzoyl leucomethylene blue (BLMB). Reference is made to U.S. Pat. No. 3,330,691 to Adams et al. Upon application of printing pressure to the assembly, the capsules ruptured and their contents were adsorbed on the clay particles disseminated on the CF sheets. As a result of an electron donor-acceptor reaction between acidic sites on the particles of attapulgite clay and the CVL, a dark blue mark was formed essentially instantaneously. This image faded rapidly and was replaced when the more slowly reacting BLMB was converted to colored (blue) form by a combined hydrolysis-oxidation mechanism. One of the problems with this type of record material was that the receiving sheets tended to lose sensitivity to the dye precursors during storage. Another was that the receiving sheets lacked the whiteness and brightness of conventional printing paper. Furthermore, aqueous suspensions of attapulgite clay were highly viscous and conventional high speed paper coating techniques could not be used.
The necessity for using both primary and secondary dyes also had obvious defects. The use of the secondary dye added to the cost of manufacturing the carbonless copying paper and it necessitated careful balancing of the relative proportions of primary and secondary dyes to assure that an image was present during the transition between fade of the CVL and full development of the BLMB. However, even with judicious selection of proportions of the dye precursors, color changes were evident throughout the life cycle of the developed sheets.
Some but not all of the drawbacks of the attapulgite pigment system have been obviated by sensitizing sheets with a pigment obtained by leaching a dioctahedral montmorillonite clay with mineral acid. The acid treatment of the crude clay increases the intensity of the image developed by CVL and it converts the clay from a material having rheological characteristics completely unsuitable for paper coating to a product which can be coated at higher solids than the attapulgite pigment. Acid-treated dioctahedral montmorillonite, exemplified by the product supplied under the tradename "Silton," has supplanted attapulgite clay as the most widely used inorganic pigment sensitizer for carbonless copying paper. It is well known, however, that CVL produces a fugitive image when developed on acid-treated dioctahedral montmorillonte such as Silton and, as shown in U.S. Pat. Nos. 3,622,364 and 3,753,761 to Sugahara et al, BLMB must be used along with the CVL. Even when using this dye precursor combination, there is a considerable change in color in the printed sheets since CVL fades from deep blue to green when it is developed on Silton. Certain solutions of carbinol dye precursors have been shown (South African Pat. No. 72/1193 to Baxter) to produce essentially stable images on montmorillonites leached with sufficient acid to increase the hydrated silica content and to destroy partially the mineral lattice. Examples of such solutions are those obtained by dissolving Michler's hydrol, derivatives of Michler's hydrol such as the benzyl ether and morpholino compounds in polar solvents of low volatility. According to the teachings of the South African patent, the dyes are stabilized by penetration of the solution into the lattice of the mineral structure. To the best of our knowledge, however, acid-leached montmorillonites capable of producing stable images with CVL are unknown and, further, the rheological properties of coating mixtures prepared using sufficiently high contents of such pigments to give adequate imaging performance are only marginally adequate for use with conventional high-speed blade coating techniques for coating paper webs.
A wide variety of other inorganic pigments has been proposed for use in carbonless copying paper and many have been reported to produce images of high intensity when placed in absorptive contact with solutions of basic chromogens such as CVL. Among such pigments are: hydrothermally treated kaolin clay (U.S. Pat. Nos. 3,223,546 and 3,226,252 to Hemstock; U.S. Pat. No. 3,723,174 to Swanson et al; U.S. Pat. No. 2,736,285 to Miller); acid-leached calcined clays such as kaolin, montmorillonite or attapulgite (British Pat. No. 1,307,319); clays such as the aforementioned calcined above 200.degree. C; preferably between 500.degree. to 700.degree. C. (French Pat. No. 1,589,266 to Hayashi et al); and lattice-expanded kaolinite (U.S. Pat. No. 3,520,719 to Horton). Insofar as we are aware, none of these pigments reacts with CVL to produce images that are sufficiently resistant to fade to be useful with transfer sheets prepared with CVL without secondary dye precursors. In this regard, it is noted that binders and dispersants used in paper coating compositions (coating colors) generally adversely affect pigment sensitivity. Therefore, pigments which are promising as image-formers when tested in cake form may be entirely unsatisfactory when formulated into coating colors and coated on paper.
An increasing amount of the carbonless copying paper that is being supplied to the market is based on the use of a solid acidic resin, usually a phenolic resin. The phenolic resins, although expensive, may be used at low levels since they are very reactive and are normally made into coating mixtures with large quantities of kaolin coating clay, hence the designation "K-P" or kaolin-phenolic system. The rheological properties are therefore good and high solids blade coating is possible. Certain K-P CF sheets produce images of acceptable stability with CVL and thus BLMB or other slowly reacting secondary dyes may be excluded from the system. However, the resins are stringent in their requirement of the precursor solvent, which must also be a solvent for the resin. The original and most suitable solvents in this regard were chlorinated aromatic hydrocarbons. A measure of the stringency of the requirement on solvents is the fact that since the chlorinated aromatics were banned for use in this application, no equivalent substitutes have been discovered. Solvents commonly used in current practice are mixtures of aliphatic, alicyclic and/or aromatic hydrocarbons, which perform adequately in this respect but are not as suitable as the chlorinated aromatics. Furthermore, the resins become oxidized and polymerized to give quinones and other products, and the coated sheets generally turn yellow and develop an offensive odor on storage. In addition, they may cause frequent washups on offset presses due to their partial solubility in alcohol-containing fountain solution, resulting in resin buildup on the press. Furthermore, the resin-coated sheets may not be used as broke on a paper machine due to the presence of the resins.
Accordingly, a general object of our invention is to provide novel substrates for carbonless copying paper systems which possess the desirable image-developing characteristics of resin-based substrates without their undesirable properties. A more specific object is to provide novel mineral-based inorganic pigment-coated substrates for record material that obviate defects of prior art substrates of this type. A primary objective is to provide substrates which produce intense relatively stable images with CVL and do not require the use of secondary dye material. Still another object is to provide substrates that are useful with other commercially used dry precursor systems.
Other objects and features of the invention will be apparent from the description which follows.